Conventional automobile brake devices are disc brake devices or drum brake devices that use a brake pad or brake shoe as a friction member which is formed by affixing the friction material on a metal base member.
The friction material especially used in the brake pad may be classified into three types, i.e., semi-metallic friction material including a steel fiber as a fiber base material, low steel friction material including a steel fiber in a part of the fiber base material, and Non-Asbestos-Organic (NAO) friction material not including steel fiber as the fiber base material.
The semi-metallic friction material is characterized by having a high wear resistance; the low steel friction material is characterized by having highly effective braking; and the NAO friction material is characterized by having a minimal brake noise occurrence.
In recent years there is a demand for minimal brake noise occurrence, therefore the NAO friction material with no steel fiber has widely been used in the brake pad. The NAO friction material includes a fiber base material, such as nonferrous metal fiber, organic fiber, and inorganic fiber; a binder, such as thermosetting resin; and a frictional wear modifier, such as an organic filler, an inorganic filler, an inorganic abrasive material, a lubricant, and a metal particle and so on.
The NAO friction material minimizes brake noise occurrence, but on the other hand, the use of the NAO friction material decreases the brake effectiveness because a thin water film forms between the friction surface of the friction material and the friction surface of the mating member when water breaks into a gap between both friction surfaces, i.e., the use of the NAO friction material causes water fade.
The Japanese Provisional Patent Publication No. 2007-191010 (Patent Document 1) shows a method of dissolving water fade by controlling the pressure on the friction material against the brake disc.
However, the method of the Patent Document 1 requires a highly precise pressure control mechanism, and therefore a technology to inhibit water fade using improved friction members, such as the brake pad, and without the use of the pressure control mechanism, was demanded.
Water fade can be inhibited by creating sufficient roughness on the friction surface of such as the brake disc or the brake drum. The sufficient roughness reduces the water surface tension, thereby making it difficult to form a thin water film.
The semi-metallic friction materials and low steel friction materials including steel fiber with a large abrasive force, creates sufficient roughness on the friction surface of the brake disc or brake drum, and therefore water fade occurrence is minimized.
However, there is a problem when using NAO friction materials with no steel fiber and a small abrasive force in that it is difficult to create sufficient roughness on the friction surface of the brake disc or brake drum and therefore water fade tends to occur.
In order to increase the abrasive force, the inorganic abrasive material to be applied in the friction material may be hardened; the particle diameter may be enlarged; or additives may be increased. However, a problem of failing to meet other required performance arises because these changes of the friction material contribute to lowering the wear resistance and heighten the aggressiveness against the mating surface (aggressiveness against the surface of the mating member).
The Japanese Provisional Patent Publication No. H08-2278866 (Patent Document 2) shows a brake pad that improves drainage and inhibits water fade occurrence by using multiple slits on the friction surface of the brake pad, where the slits are extending from an approximately center line to both ends in a longitudinal direction of the friction material and inclining in a frictional direction of the mating member.
However, in this document, specially designed slots are required, which increase the manufacturing steps.
Also, the Japanese Provisional Patent Publication No. 2007-326999 (Patent Document 3) shows a friction material including a high hardness inorganic abrasive material, and a friction material including an inorganic abrasive material, 5 volume % of graphite, and 0.5-3 volume % of coke is shown in the embodiment.
However, effective inhibition of water fade cannot be expected in the friction material in the Patent Document 3.
This is due to an excessive amount of graphite addition. Graphite has a crystal structure with laminated two-dimension graphite sheet layers of honeycomb structure, and when a shearing force is applied it generates interlaminar slide indicating fine lubricant function; therefore, graphite has widely been used as a lubricant of the friction material. However, graphite is crystalline and tends to grow a film on the friction surface of the mating member and thus increases the thickness thereof.
This thickened graphite film hinders the abrasive function of the inorganic abrasive material, and therefore creating sufficient roughness on the friction surface of the mating member cannot be realized, which makes it easy to form the thin water film and therefore water fade tends to occur.
The Japanese Provisional Patent Publication No. 2004-155843 (Patent Document 4), Japanese Provisional Patent Publication No. 2007-056063 (Patent Document 5), and Japanese Provisional Patent Publication No. 2007-112952 (Patent Document 6), disclose the invention of friction materials made of the low steel friction material, and where petroleum coke is added to the graphite as lubricant to heighten the wear resistance.
However, the low steel friction material rarely has a problem of water fade occurrence, and there is no explanation as to the relation between the friction material composition and water fade.
[Patent Document 1] Japanese Provisional Patent Publication No. 2007-191010
[Patent Document 2] Japanese Provisional Patent Publication No. H08-227866
[Patent Document 3] Japanese Provisional Patent Publication No. 2007-326999
[Patent Document 4] Japanese Provisional Patent Publication No. 2004-155843
[Patent Document 5] Japanese Provisional Patent Publication No. 2007-056063
[Patent Document 6] Japanese Provisional Patent Publication No. 2007-112952